Common Stack Patterns
The topic discusses two essential stack patterns useful in coding interviews: the monotonic stack and iterative depth-first traversal (DFS). The monotonic stack efficiently finds the next greater or smaller elements in an array by maintaining a sorted order, achieving O(n) time complexity. It is particularly applicable when problems involve comparisons of neighboring elements. Iterative DFS replaces recursive calls with an explicit stack, allowing for controlled traversal of trees or graphs, also with O(n) time complexity. Recognizing the right pattern based on problem keywords is crucial for effective problem-solving in interviews.
Recognizing a stack as the right data structure is only the first step. The next step is knowing which pattern to apply. This lesson covers the two most important stack patterns in coding interviews: the monotonic stack and iterative depth-first traversal. Each solves a specific class of problems that would otherwise require significantly more expensive approaches.
Monotonic stack
The monotonic stack pattern maintains a stack in sorted order, either strictly increasing or strictly decreasing, as we scan through an array. When an incoming element violates the sort order, we pop elements off the stack until the order is restored, processing each popped element as we go. This gives us